An isoprene polymer may have any one or two or more of the following four different structural units. That is, these structural units are: a 3,4-bond structural unit represented by Formula (I′); a trans 1,4-bond structural unit represented by Formula (II′) below; a cis 1,4-bond structural unit represented by Formula (III′); and a 1,2-bond structural unit represented by Formula (IV′)

The following two reports have been made on productions of isoprene polymers that selectively have, among the above-mentioned structural units, the structural units represented by Formula (I′). One is a report on production of the above-mentioned polymer by polymerizing isoprene using an aluminum alkyl-titanium alkoxide (AlEt3-Ti(OC3H7-n)4)—based catalyst as a polymerization catalyst (see Non-Patent Document 1). The other is a report on production of the above-mentioned polymer by polymerizing isoprene using a complex in which sparteine is coordinated to FeCl2 as a polymerization catalyst (see Non-Patent Document 2).
Meanwhile, the above-mentioned isoprene polymer that selectively has the structural units represented by Formula (I′) is considered to have significantly different properties depending on the tacticity of an arrangement of the structural units. Like an isoprene polymer including the structural units represented by Formula (I′), a polymer in which an atom of a main chain has two different side chain substituents, may have two kinds of stereoisomer. The “tacticity” means a manner or order of arrangement of suchmoieties involved in stereoisomerism in a main chain of a polymer. A polymer, in which one type of substituents of different two types of side chain substituents are bound only to one side with respect to the plane formed by the main chain of the polymer, is referred to as an isotactic polymer (represented by Formula (V) below), while a polymer, in which one type of substituents are alternately bound to the both sides with respect to the plane, is referred to as a syndiotactic polymer (represented by Formula (VI) below). Meanwhile, a polymer not having such regularity is referred to as an atactic polymer.

The above-mentioned Non-Patent Document 1 describes that the resultant isoprene polymer is an atactic polymer, while Non-Patent Document 2 does not describe the tacticity of the resultant isoprene polymer. Therefore, an isoprene polymer that selectively has Formula (I′) described above and has high tacticity has been required.                Non-Patent Document 1: Makromolekulare Chem. (1964), 77, pp. 126-138.        
Non-Patent Document 2: Macromolecules (2003), 36, pp. 7953-7958.